FIELD OF THE INVENTION
The present invention relates to an impact type clamping apparatus such as an impact wrench and an impact type nut runner, and more particularly to an impact type clamping apparatus in which a clamping force with which a fastening member such as bolt and nut is clamped can be detected accurately.
An impact wrench having a clamping force controlling faculty has been known and has been described in, for instance U.S. Pat. No. 5,181,575 issued on Jan. 26, 1993. In this known impact wrench, a main spindle is made of material having a magneto strictive effect and a distortion of the main spindle is detected by a pair of coils arranged in opposition to the main spindle to measure a pulsatory torque applied to the main spindle. When the detected pulsatory torque exceeds a predetermined threshold value, a control circuit supplies a cut-off signal to a shut-off valve so that a supply of a compressed air to an air motor unit is stopped. Then, the operation of an oil pulse generating unit producing the pulsatory torque is stopped and the rotation of the main spindle is stopped.
In a torque wrench such as a taper beam wrench, a clamping force actually applied to bolts and nuts is in proportion to a clamping force produced by the main spindle. However, in the above mentioned impact wrench, a peak value of a pulsatory torque is not in proportion to the clamping force actually applied to bolts and nuts. For instance, even when a pulsatory torque having a peak value smaller than that of a preceding pulsatory torque is generated, it has been experimentally confirmed that a clamping force with which the bolts and nuts are actually clamped is often increased. In this manner, the peak value of the pulsatory torque is not proportional to the actual clamping force, so that even if the peak value of the pulsatory torque is detected in an accurate manner, it is impossible to detect the actual clamping force accurately and the actual clamping force could not be controlled precisely by controlling the shut-off valve in accordance with the detected peak value of the pulsatory torque.